As a power generation method without emission of carbon dioxide, which is considered to be effective to reduce global warming, solar power generation and wind power generation have attracted attention in recent years.
In the solar power generation, a generated DC power is converted into an AC power of a desired frequency through an inverter. The AC power is boosted to a voltage of a commercial power system by a boosting transformer and then connected to a commercial power network. Also in the wind power generation, a generated AC power is converted into a DC power, and the DC power is further converted into an AC power of a desired frequency through an inverter, thereby contributing to enhancement of power generation efficiency.
In the solar power generation, an amount of generated power varies depending on weather conditions, the temporal change of the altitude of the sun, and so on. In the wind power generation, a power generation amount varies depending on wind speed changing momentarily. Thus, with respect to the varying power generation amount, when the DC power is converted into an AC power by an inverter and further boosted to a given voltage of a commercial power system by a boosting transformer, various control circuits are required. Such an inverter, control circuit, boosting transformer, and the like are collectively generally called a power conditioner.
A power conditioner used in the solar power generation and the wind power generation is designed in view of variation of the power generation amount within the year and daily variation of the power generation amount. However, in actual operation, a time for which the rated power generation amount is obtained is a portion of all operating time, and it is often operated in an output band less than the rated output. For example, in the solar power generation, it is considered that the largest power is generated in an output band which is 30% to 70% of the rated output (% with respect to the rated output) (see, for example, Japanese Patent Application Laid-Open (JP-A) Nos. 2010-273489 and 2012-120251).
As a boosting transformer, a transformer using a silicon steel sheet in a magnetic core has been conventionally used. However, as described above, in actual operation, the time for which the rated power generation amount is obtained is short, and degradation of conversion efficiency in the output band less than the rated output has been a problem. In association with such a situation, JP-A Nos. 2010-273489 and 2012-120251 propose a technique in which an amorphous transformer using a magnetic core including stacked Fe-based amorphous alloy thin strips considered to have high energy conversion efficiency compared with a transformer using a magnetic core formed of a silicon steel sheet is adopted in a region with a low load rate, thereby increasing the efficiency of a power conditioner.
A transformer may be held in a so-called residual magnetization state in which the transformer is maintained in a magnetized state by shutdown of an inverter or the like. In this state, the transformer easily reaches magnetic saturation in resuming operation, and normal operation cannot be performed.
In the prior art, in order to prevent occurrence of the magnetic saturation in a power conditioner, a current or voltage input to the input side (primary side) of a boosting transformer and a current or voltage output from the output side (secondary side, boosting side) are detected, and a control circuit is disposed to prevent the magnetic saturation.
For example, as an example of the control circuit, JP-A Nos. 2010-273489 and 2012-120251 disclose a power conditioner which has a function of performing offset correction before start of operation.
However, the control circuit is complex and, in addition, should be designed in accordance with the characteristics for every magnetic core of each transformer, and therefore, the control circuit has a problem in terms of versatility and simplicity.
In addition to the above disclosures, there is a disclosure regarding a transformer which can avoid magnetic saturation even when DC biased magnetization occurs, by having predetermined iron loss (see, for example, JP-A No. 2008-177517).
Further, JP-A No. 2008-177517 discloses that magnetic resistance is increased by annealing without applying a magnetic field to increase magnetic resistance, thus reducing magnetic saturation. Furthermore, JP-A No. 2008-177517 describes that the magnetic resistance is increased by annealing at a low temperature of not more than 300° C., thus reducing the magnetic saturation.